xni EESPIEATOEY EHYTHM 469 



independent of external stimuli. It is also legitimate to suspect 

 that the rhythmical activity of the isolated bulb may be main- 

 tained by irritation from the sections and the external agents 

 acting on the surface of these sections. We must now turn to a 

 striking series of facts which show that the rhythmical activity of 

 the respiratory centres is influenced in great measure, besides the 

 afferent stimuli by the nerve paths, by the condition of the blood 

 and lymph that are circulating in them, i.e. by the nature of the 

 medium which bathes the nervous elements of which they are 

 constituted. 



IX. Under all the varied circumstances, natural or experi- 

 mental, in which there is an abnormal rise of venosity of the 

 blood in consequence of the diminished gaseous exchanges between 

 the environment and the organism, dyspnoea is produced, i.e. 

 increased intensity and frequency of the respiratory rhythm. 

 This occurs regularly : 



(a) When the animal (or man) is forced to breathe an atmo- 

 sphere surcharged with carbonic acid ; 



(6) Or an atmosphere poor in oxygen and rich in indifferent 

 gases ; 



(c) When under any morbid conditions (pneumonia, pleuritic 

 effusions, pneumothorax) the alveolar respiratory surface is 

 abnormally diminished ; 



(d) When owing to uncompensated organic lesions of the heart 

 there is an abnormal retardation of circulation ; 



(e) When, lastly, owing to profuse haemorrhage, or copious 

 bleeding, the mass of blood in circulation is largely diminished. 



Since the respiratory movements are destined by the ventila- 

 tion of the lung to provide for the normal gas exchanges between 

 the atmosphere and the blood, and indirectly between the blood 

 and the tissues, these facts show that the amplitude of the 

 respiratory movements, i.e. the degree of pulmonary ventilation, 

 increases with the need for increased elimination of C0 2 and 2 

 absorption. Accordingly, there is a certain degree of adaptation 

 between the gas exchanges and the respiratory activity, which 

 implies that either the carbonic acid, or other waste products of 

 the tissues avid of oxygen, act as direct stimuli to the respiratory 

 centres, or at least modify their metabolism so as to increase their 

 rhythmical and alternate function. 



The theory of adaptation between the need for air and the 

 respiratory magnitude of ventilation is confirmed by the fact that 

 there is exaggerated activity of the respiratory centres, and there- 

 fore of the depth and frequency of respiration, whenever the 

 organic processes of combustion, i.e. the consumption of oxygen 

 and production of carbonic acid, are increased. The most classical 

 example of this fact is, under normal conditions, the dyspnoea 

 developed in consequence of intense muscular work. The influence 



